He knew that the Black Order would attack Wakanda searching for the fabled Infinity Gem. Wakanda barely survived, and neither Wakanda or Atlantis have forgiven each other from the past. There is no end in sight to their war.

Was Wakanda destroyed?

During the war between the Avengers and the X-Men, T'Challa allowed the Avengers to use Wakanda as their home-base. When the Avengers kidnapped the X-Man Transonic, Namor, possessed by the Phoenix Force, unleashed a massive tsunami which destroyed most of Wakanda.

Does Wakanda fall?

Not only did Wakanda lose half of its population like the rest of the world, but their losses were even greater after the was against the Outriders. The city was under attack and the losses stacked up. With the snap taking both T'Challa and Shuri away, Wakanda would suddenly be without two of its brightest minds.